1. Field of the Invention
The present invention relates to a technique for designing fluidic sensing circuits that exhibit constant performance characteristics independent of power supply noise or drift.
2. Description of the Prior Art
Designers of high sensitivity, high dynamic range, open loop, fluidic systems are confronted with obstacles in obtaining constant performance due to power supply noise and drift. One of the major problems from obtaining constant performance in laminar fluidic systems concerns the null offset of jet deflection sensors and amplifiers. Jet flows within a fluidic amplifier should be such that equal output pressures are obtained in both its output ports when there is no difference at its control ports. However, due to various nonlinearities in the device itself, a difference in output pressure is often seen even though there is no difference in pressure at the input or control ports. This difference in output, in the absence of a difference in pressure at the input, is called null offset. This null offset varies as the supply flow to the particular device varies. As a result of this null offset characteristic in laminar flow fluidic devices, signal errors are produced by supply noise and pressure variations, which for high gain systems can be quite large. The use of a highly regulated pressure supply would be desirable but is extremely difficult to attain and is cost prohibitive.
Previous attempts to overcome the null offset problem in sensors and amplifiers have included better fabrication techniques with the purpose to eliminate geometrical imperfections in the devices. A fine blanking technique described in Mon, U.S. Pat. No. 4,253,495, has been used to eliminate such imperfections, however, these devices still exhibit a null offset due to the difficulty in producing a perfectly symmetrical device. Consequently, signal errors are still produced when the supply flow varies.
Another technique to reduce null offset in fluidic systems is described in Manion et al. U.S. Pat. No. 4,369,811. This technique uses a trim circuit utilizing two resistors at the control points of the device. Nevertheless, there exists a need for fluidic systems that can sense a variety of parameters such as temperature, gas concentration, viscosity, density etc., and that are insensitive to power supply noise and pressure variations. The use of null balance, closed loop systems to suppress similar disturbances is well known in the field of electronics but has not been devised for implementing this concept in the field of fluidics.